Functional organization in electroreceptive midbrain of the catfish

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Abstract
The response properties of electroreceptive units in the torus semicircularis (TS) of the catfish (Ictalurus nebulosus) were analyzed for evidence of functional organization. The effects of both frequency and spatial parameters of electric-field stimuli were investigated. Uniform electric fields were used in studying response latencies, field-gradient orientation preferences and frequency tuning. Weak dipole electric fields were used in studying unit receptive fields and selectively for direction and speed of stimulus movement. The field-gradient orientation preferences of single units rarely corresponded to that of the evoked potential recorded at the same locus. Receptive fields could be found for 77% of the electroreceptive units; 3 types of receptive fields were distinguished. The receptive fields of units in the same dorsoventral electrode track were superimposed and thereby maintained the somatotopic relationship of contralateral receptive fields found in the rostrocaudal axis of the nucleus. Unit-response latencies decreased with increasing unit depth in the dorsoventral axis. Units located dorsally in the TS were tuned to a lower range of frequencies (0.05-10 Hz) than those located ventrally in the TS (4-15 Hz). Two types of action potentials (AP) having conspicuously different time courses were recorded in discrete areas of the TS: AP with a 1 ms duration were located dorsally and make up 90% of the units sampled; AP 0.5 ms in duration were clustered at the ventral margin of the nucleus. Because of several differences in their respective response properties, it is proposed that the fast and slow units represent functionally distinct neuronal populations. Functional organization in the rostrocaudal axis of the TS appears to be based simply on the somatotopic relationship of unit receptive fields, while functional organization in the dorsoventral axis is most parsimoniously explained if known unit subtypes in the medulla project in parallel and discretely to dorsal and ventral portions of the TS.